The present invention relates to an apparatus for recording information on a recording medium by holography, an apparatus for reproducing the information from the recording medium, and a method for adjusting incident angle of reference beam.
At the present time, capacities of optical disks exceeding 100 GB have been attained based on BDXL (trade mark) format. In the future, further improvement of capacity will be required for optical disks due to tremendous growth of the field of information technology. In many research institutes, study has been made on next-generation storage techniques. As one of the storage techniques, attention is focused on a holographic recording technique. The holographic recording technique is a technique for recording information by overlapping signal beam having two-dimensionally modulated page data by a spatial light modulator with reference beam in a recording medium, and by producing refractive index modulation within the recording medium due to an interference pattern generated in the medium at the time. Since a plurality of pieces of page data can be multiple-recorded at the same location of the recording medium, higher-speed recording and a larger capacity can be attained. Up until now, a plurality of multiple recording techniques have been proposed, and a typical one of such techniques is angle multiplex recording. In an apparatus based on the angle multiplex recording, precise adjustment of an incident angle of reference beam becomes important. In US2009/0207710, for example, there is disclosed a method of, upon reproduction of a series of angle multiplex holograms, estimating an incident angle optimum for reproduction of a next hologram by use of a reproduction result of one of the holograms just before thereto or before by several ones.
JP-2009-54214, further, describes in Paragraph 0025 and so on that “When a beam spot is moved along a direction crossing an array direction of light receiving elements, an angle detection signal of the reference beam varies within a range between a certain constant positive potential and a negative potential. That is, the level of the angle detection signal of the reference beam varies according to the incident angle of the reference beam during recording. With such an arrangement, when the reference beam is applied so that the center of the beam spot is located on boundaries of mutually-adjacent light receiving elements of an optical detector 32, the reference beam angle detection signal has a level of zero. And, for example, the zero-cross point can be associated with the positioning point of the reference beam angle. In this case, for example, when the angle of the reference beam is suitable, the level of the reference beam angle detection signal becomes zero. When the level of the reference beam angle detection signal is zero or in a predetermined range within the vicinity of zero, the main controller 300 determines that the angle of the reference beam to be adequate.”